Manganese Catalysts

The manganese catalyst refers to an elemental manganese or manganese compound which has a catalytic function. Manganese is an off-white transition metal, which is relatively active. It can combine with oxygen when heated, and is easily soluble in dilute acid to form divalent manganese salt. The valence of manganese is 0, +2, +3, +4, +5, +6 and +7. Among them, +2, +4 and +7 are common valences. Manganese is abundant in the earth's crust, so manganese catalysts are abundant and cheap.

Applications:

Because manganese catalyst has the advantages of low price, good catalytic effect and environmental friendliness, it can catalyze many types of reactions in organic synthesis, and also plays an important role in the field of environmental protection.

• Friedel-Crafts alkylation: In the polyoxo complex of low-valent manganese, the π-acidity of the carbon monoxide ligand causes the metal center to be in an electron deficient state and thus has a certain degree of Lewis acidity. Like other metal Lewis acids, these low-valent manganese complexes act as activating reagents to promote the electrophilic substitution of aromatic hydrocarbons and the nucleophilic addition of polar unsaturated compounds. For example, manganese pentacarbonyl bromide can catalyze the Friedel-Crafts alkylation of anisole. In addition, some inorganic manganese compounds have unique catalytic activities and can also catalyze the Fischer-Craft alkylation reaction. For example, manganese dichloride tetrahydrate can catalyze the electrophilic substitution reaction of hydrazine with 3,4-dihydropyran to form various 3-position alkylated anthracene derivatives.

Figure 1. Manganese catalyst catalyzed Friedel-Crafts alkylation

• Coupling reaction: The Suzuki-Miyaura coupling reaction of halogenated aromatic hydrocarbons and arylboronic acids is an important method for the synthesis of biaryl compounds. The supported manganese catalyst can be prepared by introducing manganese ions into the structure of apatite (HAP) to form a catalytic active center. This supported manganese catalyst can catalyze the Suzuki cross-coupling reaction of arylboronic acid and brominated aromatic hydrocarbons. The complex formed by the manganese catalyst with some of the ligands also can catalyze the coupling reaction. For example, the amino acid triazole manganese formed by metal manganese and an amino acid triazole ligand can catalyze the coupling reaction of phenylhydrazine methide.

Figure 2. Manganese catalyst catalyzed coupling reaction

• Acylation reaction: The organomanganese reagent can be synthesized by a metal transfer reaction of an organolithium reagent or an organomagnesium reagent with a manganese dihalide in an ether solvent. Various types of organomanganese reagents [R_nMn(II): n = 1, 2, 3, 4] can be prepared by adjusting the ratio of organolithium (organomagnesium) reagent to manganese salt. These organomanganese compounds can catalyze the acylation of a Grignard reagent with an acid chloride. As a catalyst, organic manganese can not only produce ketones in high yield and high chemical selectivity, but also avoid the formation of by-product tertiary alcohols.

Figure 3. Manganese catalyst catalyzed acylation reaction

• C-H activation: C-H activation can be used to construct C-C bonds or C-X bonds directly through C-H bonds. Manganese-catalyzed C-H bond activation has the advantages of low cost, low toxicity, good selectivity, atomic economy and environmental friendliness. Up to now, various sp2 or even sp3 hybrid C-H bond activations can be achieved with manganese catalysts. For example, pentacarbonyl manganese bromide as a catalyst can catalyze the insertion of an aldehyde to a 2-phenylimidazole C-H bond. Mn(CO)₁₀ as a catalyst can catalyze the allyl carbonate to achieve C-H allylation.

Figure 4. Manganese catalyst catalyzed C-H activation

• Environmental protection: Volatile organic compounds (VOCs) are mainly derived from industrial processes and automobile exhaust emissions, and are highly hazardous to the environment. Catalytic oxidation is considered to be one of the most promising technologies for the treatment of volatile organic compounds due to its high destruction efficiency. Manganese oxide has strong reducibility and ability to store/release oxygen and is considered to be the most promising transition metal oxide catalyst. Manganese oxide is widely used as a catalyst for the catalytic oxidation of volatile organic compounds such as toluene, formaldehyde, and the like. In addition, the supported manganese catalyst can be prepared by loading the manganese catalyst onto the activated carbon, and can be used for catalytic degradation of substances such as methylene blue.

References

1. Yang, Zhenjie. (2019). "Effect of Calcination Temperature on the Characteristics and Performance of Solid Acid WO₃/TiO₂-Supported Lithium-Manganese Catalysts for the Oxidative Coupling of Methane." Asian Journal of Organic Chemistry 8(3), 376-384.
2. Schneekoenig, Jacob. (2019), "Manganese Catalyzed Asymmetric Transfer Hydrogenation of Ketones Using Chiral Oxamide Ligands" Synlett 30(4), 503-507.
3. Liu, Bingxian. (2018), "Divergent Annulative C-C Coupling of Indoles Initiated by Manganese-Catalyzed C-H Activation" ACS Catalysis 8(10), 9463-9470.